The present invention relates to a sheathed glow plug used in starting aids for internal combustion engines such as diesel engine, and in sheathed heater, liquid heating devices and the like, and to a method of manufacturing the same.
Various proposals are made for a sheathed glow plug comprising a cylindrical main metal shell, a heat resisting tube mounted in a leading end of a through hole of the main metal shell, a center electrode disposed in a center portion of the through hole of the main metal shell and an electric heat generator electrically connected to the center electrode, which is accommodated in the heat resisting tube and disposed between the center electrode and the heat resisting tube, wherein the center electrode is composed by coaxially welding a leading end axial shaft and a rear end axial shaft.
As shown in FIG. 9, hitherto, a center electrode (a) used in the sheathed glow plug is formed by welding a leading end axial shaft (b) and a rear end axial shaft (c) whose connecting end""s diameter is the same as that of the former axial shaft such that their connecting ends are mutually coaxial. By the way, in case that their butting faces are not correctly at right angle with respect to an axis, a coaxial property between both axial shafts is deteriorated. Further, as shown in FIG. 9, if there is a tiny protruded portion (d) on a connecting/contacting face, the protruded portion (d) first contacts and a melting is concentrated to this portion. Therefore, a molten metal flows to this portion and thus a deviation of bonded state in a face direction occurs and, thereby, such a case occurs that a regular face-connecting is not performed and this becomes a cause of an eccentricity as well.
On the other hand, recently, a diesel engine of direct injection type becomes a main stream and, corresponding to this, a glow plug whose length is capable of reaching a combustion chamber of the engine has been demanded. By the way, if the glow plug is made long in this manner, it becomes impossible to neglect a minute eccentricity generated when the leading end axial shaft and the rear end axial shaft are bonded, and it follows that there is generated such a case that the glow plug contacts with an inner face of the through hole of the main metal shell, so that a short-circuit occurs. Therefore, it has become a recent important problem to perform the connecting between both axial shafts as regularly as possible without the eccentricity
Additionally, by mutually welding faces of connecting end of axial shafts (a) and (b), a molten metal protrudes in a periphery of that connecting portion to form a burr (x) (refer to FIG. 10). If the burr(x)is left as it is, since the burr (x) contacts with an inner face of a main metal shell to generate an electric short-circuit, it is necessary to remove the burr by means such as a grinder. By the way, hitherto, a maximum size of the burr becomes larger in its diameter than the axial shaft. And, in order to ensure an insulation between an inner face of the through hole of the main metal shell and the center electrode, it is necessary to approximately completely remove the burr, so that a time is required in the removal process. Further, in case that the burr removal is performed by the grinder, since a work is magnetized, it is necessary to perform a demagnetization treatment, so that there have been problems that a process is increased, the connecting face periphery is shaved and becomes brittle, and a strength is lowered.
An object of the invention is to solve the problems of such a conventional constitution.
The invention is a method of manufacturing a sheathed glow plug, the glow plug comprising: a cylindrical main metal shell; a heat resisting tube mounted to a leading end of a through hole of the main metal shell; a center electrode, that comprises a leading end axial shaft and a rear end axial shaft coaxially welded each other, disposed in a center portion of the through hole of the main metal shell; and an electric heat generator received in the heat resisting tube, the electric heat generator having one end electrically connected to the center electrode, which comprises a process of manufacturing the center electrode comprising the steps of: making a diameter of a connecting end of one of the axial shafts smaller than a diameter of a connecting end of the other axial shaft; disposing the leading end axial shaft and the rear end axial shaft on the same axis; bringing the connecting ends into contact; and connecting them by a resistance welding after bringing of the connecting ends.
In such a constitution, the butting face between the connecting ends becomes a small area depending on the diameter of the connecting end having a smaller diameter and, after such butting face has been preferentially molten, the other portion is welded. Therefore, even if a connecting end face has an error of right angle degree or becomes a rough face, its influence can be suppressed because the diameter of the butting face is small, so that the resistance welding can be stably performed. Further, even if a burr is generated in a periphery edge of the connecting face by the resistance welding, a burr generation amount becomes small because an area of the butting face is small. And, so long as the burr does not protrude the main diameter of the axial shaft having a larger diameter, it becomes an allowable range, so that a burr removable can be omitted or a simple burr removal work suffices.
As a constitution capable of achieving a mutual diameter relation of the connecting ends, there is proposed a constitution in which the connecting end of one axial shaft of the center electrode is made smaller in its diameter than its main diameter and the diameter of that connecting end is made smaller than the connecting end of the other axial shaft. In such a constitution, since it is one in which the main diameters of the axial shafts are made different and the connecting end of the axial shaft having a larger diameter is made smaller in its diameter than the connecting end of the other axial shaft, the diameter of the connecting end having a small diameter prescribing the butting face can be made as small as possible without making the diameters of both axial shafts small so much. Therefore, it is possible to regularly bond the axial shafts together without reducing a strength of the center electrode to improve a coaxial degree. Further, since the diameter of the butting face is small, the maximum size of the burr becomes small as well, so that the burr removal becomes easy or unnecessary.
As such a constitution, there is proposed one in which the connecting end is formed by forming, in a connecting side end portion of one axial shaft of the center electrode, a different diameter protruded portion having a smaller diameter than a main diameter portion, thereby making the diameter of that connecting end smaller than the connecting end of the other axial shaft.
Further, there is proposed a constitution in which the connecting end is formed in the connecting side end portion of one axial shaft of the center electrode by forming a tapering truncated cone portion, thereby making the diameter of that connecting end smaller than the connecting end of the other axial shaft. By making it into a truncated cone form in this manner, it is possible to reduce an area of the connecting end preferentially bonded and a burr maximum size becomes small. Additionally, since the diameter of the connecting portion becomes large as that end face melts, it follows that a connecting strength is increased. As this truncated cone form, there are a truncated cone form, a truncated pyramid form, and the like.
Here, in such a manufacturing method, in order to remove the burr, it is possible to remove the burr generated in a connecting portion periphery edge by more than two points argon arc welding. In such a burr removal means, the burr removal can be performed simply and without deteriorating the welding strength in comparison with a case where the burr removal is performed by a grinder.